Safety relief valves provided for boiling-water reactor power plants and other types of nuclear power plants are equipments having steam relieving functions and safety functions and constituting a main steam system. The main steam system is comprised of a main steam pipeline, a safety relief valve, a steam flow restrictor, a main steam isolation valve, a main steam pipe drain system, and a feed water system. The functions of the main steam system include a steam supply from a reactor pressure vessel to a turbine, a pressure suppression of the reactor pressure vessel within a limit value in a transient state of a reactor, and steam releasing restriction from the reactor pressure vessel and a reactor containment vessel.
The main steam system generally includes four main steam pipes for introducing steam generated within the reactor pressure vessel to the turbine. A plurality of safety valves is provided for each main steam pipe. Safety valves are provided for a main steam pipe in order to suppress reactor pressure to a value less than a specified value if, for some reason, an accident or the like occurs in the reactor or in the vicinity thereof. A safety valve has spring-operated safety functions and relief valve functions for forcibly opening the safety valve by an auxiliary actuator at a set pressure less than a blowout pressure.
As the relief valve functions, the safety valve releases steam in the reactor pressure vessel to a pressure suppression pool by means of forced manual opening or automatic opening in response to a high relief valve pressure. Some safety valves are built in an auto-depressurization system to be enabled in case of a loss-of-coolant accident (LOCA). The safety valves are automatically forced to open by means of remote operation based on a high reactor containment vessel pressure signal or a low reactor water level signal, thereby depressurizing the reactor pressure vessel until cooling water injection by a low-pressure emergency core cooling system becomes possible.
Conventional technology will be described hereunder with reference to FIGS. 4 to 7.
FIG. 4 illustrates a reactor containment vessel of a boiling-water nuclear power plant, and FIG. 5 is an enlarged view of a pressure suppression pool illustrated in FIG. 4. As illustrated in FIGS. 4 and 5, a reactor pressure vessel 21 is installed within a reactor containment vessel 14, and a safety valve 5 is provided in a pipe of a main steam system 11. In the safety valve 5, there is provided a safety valve exhaust pipe 28 for introducing steam to a pressure suppression pool 10. Vent pipes 24 are provided in a wall of the pool, and a quencher 23 for facilitating steam condensation in the pressure suppression pool 10 is connected to the lower end of the safety valve exhaust pipe 28. Note that in FIG. 4, reference numerals 22a and 22b denote main steam isolation valves.
FIG. 6 illustrates the configuration of a safety valve and FIG. 7 illustrates a safety valve drive system. As illustrated in FIGS. 6 and 7, accumulators 3 and 4 are provided conventionally to supply an operating gas from a high-pressure nitrogen gas supply system 1 (1a, 1b), in order to open the safety valve 5 if an accident or a transient state occurs. The safety valve 5 is a nitrogen- and spring-operated type and is mounted on a pipe stand provided for the main steam pipe of the reactor containment vessel. An outlet side of the valve is formed as a flange connected to an exhaust pipe. The safety valve 5 is designed to automatically open (safety functions) if a valve inlet pressure exceeds a spring load. A piston 18 disposed in an air cylinder 19 mounted on the valve main unit and a valve shaft 17 are coupled with each other by means of a pull-up lever 16. Thus, the valve is configured so as to be opened (relief valve functions) by supplying nitrogen into the air cylinder 19 using an external signal. Supply of nitrogen into the air cylinder 19 is performed by operating a controlling solenoid valve.
Next, an operating logic of the safety valve 5 will be explained. As illustrated in FIG. 7, the safety valve 5 operates, in response to a simultaneous signal of a reactor water level “low” and a dry well pressure “high”, as the result of an auto-depressurization system actuating signal 9 being generated with an emergency core system pump enabled. If one of two solenoid valves for auto-depressurization functions 26a and 26b is opened in response to logic circuit output signals from the output signal cables 34b and 34c of logic circuits, a nitrogen gas is supplied from a high-pressure nitrogen gas supply system 1 or an accumulator 4, thereby forcing the safety valve 5 to open. Consequently, steam flows from the main steam system 11, in a direction shown by arrows S1 and S2, into the pressure suppression pool 10, thereby depressurizing the reactor pressure vessel.
On the other hand, if the pressure of a reactor rises and a high relief valve pressure signal is generated by a pressure gauge 13 for relief valve functions, a signal for relief valve functions is generated through an output signal cable 34a of a logic circuit, thereby causing the safety valve 5 to operate. If one solenoid valve 25 having relief valve functions opens in response to an output signal of a logic circuit, a nitrogen gas is supplied from the high-pressure nitrogen gas supply system 1 or an accumulator 3, thereby forcing the safety valve 5 to open. Consequently, steam flows into the pressure suppression pool 10 in the same way as described above, thereby depressurizing the reactor pressure vessel.
Further, in the figure, reference numeral 14 denotes a reactor containment vessel side and reference numeral 15 denotes a reactor building side. In addition, examples of conventional proposals of such a safety valve drive system described above include one described in Patent Document 1 (Japanese Patent Laid-Open No. 9-304584).
As described above, the operating logic of a safety valve works in the manner that if at least one of three three-way solenoid valves operates, the safety valve is forced to open, thereby depressurizing the reactor pressure vessel. In addition, according to the current operating logic of a driving solenoid valve, there is a possibility that if a fire occurs, a cable short-circuits to another cable and a false signal is generated, thus causing the solenoid valve to open mistakenly. Moreover, if the safety valve opens due to the malfunction of the solenoid valve, the depressurization of the reactor pressure vessel or the outflow of reactor water occurs, thus causing the water level of a reactor to drop. In addition, it is conceivable that in current systems of safety valves, online maintenance becomes difficult to perform if a power source for driving an auto-depressurization system is lost.